1. Field of the Invention
The invention relates generally to the field of optical recording apparatus and more specifically to codes for recording data on the optical media.
2. Description of the Prior Art
The origins of the present invention lie in the optical recording environment, where it often is preferred to have a clock or pilot signal prerecorded on the optical media which is decoded by the optical system when the data is read. In order to decode a prerecorded pilot signal or clock the data must be recorded in such a manner that a null in the frequency spectrum is present upon reading the data. The frequency of the clock is chosen to coincide with the null.
One such code generating a null in the frequency spectrum was the so-called quadphase code. In this code two bits of binary code are encoded into a symbol. In optical recording, the two bits are encoded by writing holes corresponding to the bit pattern in the first half of a symbol and writing holes corresponding to the inversion of the bit pattern in the second half of the symbol. Thus every symbol has four symbol positions. A binary 00 would be written with two holes in symbol positions 1 and 2 and 2 spaces written in symbol positions 3 and 4. Binary pattern 01 would have a hole in symbol position 1 and a hole in symbol position 4 with no holes in positions 2 and 3. Binary 10 would have a hole in positions 2 and 3 and no holes in positions 1 and 4 and bit binary 11 would have no holes in positions 1 and 2 and holes in positions 3 and 4.
Quadphase coding produces a null in the frequency spectrum of the data at a frequency F.sub.o corresponding to 1/2 of the frequency of symbol positions. This frequency coincidentally is the frequency of the binary data, i.e. 4 symbol positions correspond to 2 bits of binary data.
While quadphase coding generates a null in the frequency spectrum, it was found to be inefficient with respect to laser burden: it uses one pulse per bit. An improved code was therefore developed. This code is the so-called 2/8 or two out of eight position code ("TOEP"). This code is shown in FIG. 1. This code encodes four bits of information over eight symbol positions but uses only two holes within any symbol. As with the quadphase code, it generates a null in the frequency spectrum at 2.pi./T.sub.o, where T.sub.o is the period of two symbol positions. FIG. 2 shows its power spectrum.
The code also is a so-called D=2 code wherein two positions are reserved between holes or groups of holes. Due to its D=2 nature, the TOEP code generates good read margins within a symbol, but due to the fact that D=2 is not maintained at symbol boundaries, the density of symbol positions was limited by the worst case boundary condition.
In order to increase the density of code, it was thought desirable to space the symbol positions so closely together that the size of a hole, that is the diameter of a hole, may be greater than one symbol position. This produced obvious problems when reading and decoding a symbol in that the signal due to a symbol "spreads" into adjacent symbol positions. This problem is magnified due to the optics of the laser read beams commonly employed in optical recording systems. These beam spots are not sharply defined beams but rather possess the form of a Gaussian curve, with half power widths about equal to the diameter of the holes produced in writing with the same spot. For holes or groups of holes, the half power width of a signal caused by the reading roughly corresponds to the diameter of the holes. Thus there will be considerable read signal power extending beyond the diameter of a hole.
These two problems, i.e. hole diameter versus symbol position spacing and read signal overlap, further combined to limit the bit density of optical recording with fixed block codes.